The present invention relates to a process of stimulating oil recovery utilizing carbon dioxide, and more particularly to such a process in which the carbon dioxide is in the liquid state and a back pressure of only a relatively small magnitude, from atmospheric to approximately 300 psi, is maintained on the underground formation during the production cycle.
It is now well known that there are significant reservoirs of low gravity crude oil in underground formations. Because of this, extensive efforts have been undertaken over the years to develop feasible techniques to stimulate the production of oil from such reservoirs. Since the oil which remains in such formations, however, is highly viscous, it is very difficult to recover.
A number of methods have been attempted to stimulate oil recovery from such underground formations including flooding, steam injection and gas injection, but to date none has been totally satisfactory. Thus, in most instances, for example, the viscous oil cannot be displaced efficiently by water or other flooding agents. By the same token, steam injection has certain disadvantages in that it cannot be used successfully in certain types of formations and also requires the availability of inexpensive fuel and a large supply of good water.
A variety of chemical additives have also been evaluated to enhance the flow of viscous oil, but these likewise have significant limitations. Thus, while the viscosity of the oil can be effectively reduced by diluting with appropriate solvents, the solvent must not only be soluble in the oil, but must not break out as an immiscible, mobile phase produced preferentially to the oil, or if it does break out, must then remain trapped as an immobile phase to provide energy to promote stimulated oil flow by expansion. Accordingly, it is difficult to obtain a solvent possessing such necessary characteristics. Additionally, available organic solvents are unable to penetrate sufficiently deeply into the reservoir, and, consequently, only a relatively small incremental gain in oil production is achieved.
It has also been known for a number of years that carbon dioxide is useful in stimulating oil recovery due to its high solution factor in crude oils which causes the viscosity of the crude oil-carbon dioxide solution to be markably lower than that of the crude oil itself. For illustrative examples of stimulation processes utilizing carbon dioxide, reference may be made to U.S. Pat. No. 3,442,332 and to the list of United States Patents and publication identified therein in column 2, lines 24 through 49.
In general, carbon dioxide oil recovery processes are of two types. First, where direct communication between adjacent wells exist or can be established, carbon dioxide may be introduced into the formation by one or more injection wells and the solution of crude oil and carbon dioxide withdrawn through one or more production wells which are different from the injection wells. This second method, which is generally identified as the "huff and puff" method, utilizes the same well for both injection and production purposes and is especially suitable for the recovery of crude oil where communication between adjacent wells has not been established. In this method, carbon dioxide is introduced into the underground formation, and the formation is then closed off to permit the oil to absorb the carbon dioxide. The crude oil with the carbon dioxide absorbed therein will then expand to fill the voids left by the dissolved carbon dioxide and the water displaced by introduction of the carbon dioxide into the formation. Upon releasing the closure on the formation, the expanded carbon dioxide-crude oil solution will flow or can be easily pumped to the surface where it is collected and the carbon dioxide subsequently separated from the crude oil.